Group III Nitride-Based Transistor Device

ABSTRACT

In an embodiment, a Group III nitride-based transistor device, includes a first Group III nitride barrier layer arranged on a Group III nitride channel layer, the first Group III nitride barrier layer and the Group III nitride channel layer having differing bandgaps and forming a heterojunction capable of supporting a two-dimensional charge gas. A source, a gate and a drain are on an upper surface of the first Group III nitride barrier layer. A gate recess extends from the upper surface of the first Group III nitride barrier layer into the first Group III nitride barrier layer. A p-doped Group III nitride material arranged in the gate recess has a first extension extending on the upper surface of the first Group III nitride barrier layer towards the drain. The first extension has a length ld, and 0 nm≤ld≤200 nm.

BACKGROUND

To date, transistors used in power electronic applications havetypically been fabricated with silicon (Si) semiconductor materials.Common transistor devices for power applications include Si CoolMOS®, SiPower MOSFETs, and Si Insulated Gate Bipolar Transistors (IGBTs). Morerecently, silicon carbide (SiC) power devices have been considered.Group III-N semiconductor devices, such as gallium nitride (GaN)devices, are now emerging as attractive candidates to carry largecurrents, support high voltages and to provide very low on-resistanceand fast switching times.

SUMMARY

In some embodiments, a method of fabricating a gate structure for aGroup III nitride-based transistor device comprises forming a hard maskon an upper surface of a first Group III nitride barrier layer that isarranged on a Group III nitride channel layer, the first Group IIInitride barrier layer and the Group III nitride channel layer havingdiffering bandgaps and forming a heterojunction capable of supporting atwo-dimensional charge gas, the hard mask having an opening, removing atleast a portion of the first Group III nitride barrier layer exposed inthe opening of the hard mask to form a recess having a base and sidewalls, the recess extending from the upper surface of the first GroupIII nitride barrier layer into the first Group III nitride barrier layerand forming a p-doped Group III nitride material in the recess, theopening defining the lateral extent of the p-doped Group III nitridematerial for the gate structure.

In some embodiments, a Group III nitride-based transistor device,comprises a first Group III nitride barrier layer arranged on a GroupIII nitride channel layer, the first Group III nitride barrier layer andthe Group III nitride channel layer having differing bandgaps andforming a heterojunction capable of supporting a two-dimensional chargegas, a source, a gate and a drain on an upper surface of the first GroupIII nitride barrier layer, a gate recess extending from the uppersurface of the first Group III nitride barrier layer into the firstGroup III nitride barrier layer and a p-doped Group III nitride materialarranged in the gate recess and having a first extension extending onthe upper surface of the first Group III nitride barrier layer towardsthe drain. The first extension has a length l_(d), and 0 nm≤l_(d)≤200nm.

Those skilled in the art will recognize additional features andadvantages upon reading the following detailed description, and uponviewing the accompanying drawings.

BRIEF DESCRIPTION OF THE FIGURES

The elements of the drawings are not necessarily to scale relative toeach other. Like reference numerals designate corresponding similarparts. The features of the various illustrated embodiments can becombined unless they exclude each other. Exemplary embodiments aredepicted in the drawings and are detailed in the description whichfollows.

FIGS. 1a to 1g illustrate a method for fabricating a gate structure fora Group III nitride-based transistor device.

FIGS. 2a to 2e illustrate a method of fabricating a gate structure for aGroup III nitride-based transistor device.

FIG. 3 illustrates a Group III nitride-based transistor device with agate structure according to an embodiment.

FIG. 4 illustrates a Group III nitride-based transistor with a gatestructure according to an embodiment.

FIG. 5 illustrates a Group III nitride-based transistor with a gatestructure according to an embodiment.

FIG. 6 illustrates a Group III nitride-based transistor with a gatestructure according to an embodiment.

FIG. 7 illustrates a flow chart of a method of fabricating a gatestructure for a Group III nitride-based transistor device.

FIGS. 8a to 8d illustrate a method for fabricating a gate structure fora Group III nitride-based transistor device.

FIGS. 9a to 9d illustrate a method for fabricating structures for aGroup III nitride-based transistor device.

DETAILED DESCRIPTION

In the following detailed description, reference is made to theaccompanying drawings, which form a part hereof, and in which is shownby way of illustration specific embodiments in which the invention maybe practiced. In this regard, directional terminology, such as “top”,“bottom”, “front”, “back”, “leading”, “trailing”, etc., is used withreference to the orientation of the figure(s) being described. Becausecomponents of the embodiments can be positioned in a number of differentorientations, the directional terminology is used for purposes ofillustration and is in no way limiting. It is to be understood thatother embodiments may be utilized and structural or logical changes maybe made without departing from the scope of the present invention. Thefollowing detailed description, thereof, is not to be taken in alimiting sense, and the scope of the present invention is defined by theappended claims.

A number of exemplary embodiments will be explained below. In this case,identical structural features are identified by identical or similarreference symbols in the figures. In the context of the presentdescription, “lateral” or “lateral direction” should be understood tomean a direction or extent that runs generally parallel to the lateralextent of a semiconductor material or semiconductor carrier. The lateraldirection thus extends generally parallel to these surfaces or sides. Incontrast thereto, the term “vertical” or “vertical direction” isunderstood to mean a direction that runs generally perpendicular tothese surfaces or sides and thus to the lateral direction. The verticaldirection therefore runs in the thickness direction of the semiconductormaterial or semiconductor carrier.

As employed in this specification, when an element such as a layer,region or substrate is referred to as being “on” or extending “onto”another element, it can be directly on or extend directly onto the otherelement or intervening elements may also be present. In contrast, whenan element is referred to as being “directly on” or extending “directlyonto” another element, there are no intervening elements present.

As employed in this specification, when an element is referred to asbeing “connected” or “coupled” to another element, it can be directlyconnected or coupled to the other element or intervening elements may bepresent. In contrast, when an element is referred to as being “directlyconnected” or “directly coupled” to another element, there are nointervening elements present.

A depletion-mode device, such as a high-voltage depletion-modetransistor, has a negative threshold voltage which means that it canconduct current at zero gate voltage. These devices are normally on. Anenhancement-mode device, such as a low-voltage enhancement-modetransistor, has a positive threshold voltage which means that it cannotconduct current at zero gate voltage and is normally off. Anenhancement-mode device is not limited to low voltages and may also be ahigh-voltage device.

As used herein, the phrase “Group III-Nitride” refers to a compoundsemiconductor that includes nitrogen (N) and at least one Group IIIelement, including aluminum (Al), gallium (Ga), indium (In), and boron(B), and including but not limited to any of its alloys, such asaluminum gallium nitride (Al_(x)Ga_((1-x))N), indium gallium nitride(In_(y)Ga_((1-y))N), aluminum indium gallium nitride(Al_(x)In_(y)Ga_((1-x-y))N), gallium arsenide phosphide nitride(GaAs_(a)P_(b)N_((1-a-b))), and aluminum indium gallium arsenidephosphide nitride (Al_(x)In_(y)Ga_((1-x-y))As_(a)P_(b)N_((1-a-b))), forexample. Aluminum gallium nitride and AlGaN refers to an alloy describedby the formula Al_(x)Ga_((1-x))N, where 0<x<1.

In some embodiments, a Group III nitride-based transistor device isprovided that has a gate structure including a p-doped Group III nitridelayer under a metal gate. The p-doped Group III nitride layer isarranged in a gate recess and and has an extension in the direction ofthe drain that is smaller than the extension provided by a T-shape gatein which the T-shape provides a field plate. The extension may bepositioned on an upper surface of a Group III nitride barrier layer ofthe Group III nitride-based transistor device with the gate recessextending into the Group III nitride barrier layer. The length of theextension towards the drain lies within the range of 0 to 200 nm or 10nm to 100 nm. In some embodiments, the p-doped Group III nitride layerhas no extension in the direction of the drain. A typical T-shapedrecessed gate structure, which provides a field plate, has an extensionwith a length of at least 400 nm.

Surprisingly, and contrary to the expected behaviour of a field plate, ashorter extension, or even no extension, in the direction of the drainhas been found to reduce gate leakage current, whereas lengths of 400 nmor more have been found to increase the gate leakage current. Withoutbeing bound by theory, one explanation for this behaviour is thatleakage into the gate is related to a hole current formed at theinterface between the Group III nitride barrier and an overlyinginsulation layer. This hole current is thought to flow into the p-dopedGroup III nitride gate material. Any potential difference between thechannel and the gate structure would appear to enhance gate leakage.Consequently, reducing or eliminating any potential difference betweenthe channel and the gate structure would appear to be beneficial.

In some embodiments, a p-doped Group III nitride layer has an extensionextending towards the source has a length which is greater than thelength of the extension that extends towards the drain. This arrangementis also the opposite to that commonly used in which the field plate onthe drain side of the gate is larger than the field plate on the sourceside of the gate.

FIGS. 1a to 1g illustrate a method for fabricating a gate structure fora Group III nitride-based transistor device.

FIG. 1a illustrates a Group III nitride barrier layer 10 arranged on aGroup III nitride channel layer 11. The Group III nitride barrier layer10 and the Group III nitride channel layer 11 have differing bandgapsand form a heterojunction 12 capable of supporting a two-dimensionalcharge gas. The channel layer 11 may be positioned on a substrate whichcannot be seen in FIGS. 1a to 1g . The Group III nitride barrier layer10 may comprise AlGaN and the Group III nitride layer 11 may compriseGaN.

A hard mask 13 is formed on upper surface 26 of the Group III nitridebarrier layer 10. The hard mask 13 may include silicon nitride orsilicon oxide. The hard mask 13 has an opening 14 such that a portion ofthe Group III nitride barrier layer 10 is uncovered by the hard mask 13.

At least a portion of the first Group III nitride barrier layer 10exposed in the opening 14 of the hard mask 13 is removed to form arecess 15 having a base 16 and sidewalls 17, as is illustrated in FIG.1b . The recess 15 may be formed by wet chemical etching or plasmaetching. The recess 15 may have an elongate strip-like form that extendsinto the plane of the drawing. The recess 15 may have an elongate trenchform. The recess 15 extends from the upper surface 26 of the Group IIInitride barrier laver 10 into the Group III nitride barrier layer 10. Insome embodiments, the recess 15 has a depth such that the base 16 of therecess 15 is positioned within and formed by the material of the firstGroup III nitride channel layer 11, as illustrated in FIG. 1b . In otherembodiments, such as that illustrated in FIG. 1c , the recess 15 has asmaller depth and has a base 16 that is positioned within and formed bythe material of the Group III nitride barrier layer 10.

In some embodiments, such as that illustrated in FIG. 1c , a p-dopedGroup III nitride material 18 is formed in the recess 15, whereby theopening 14 in the hard mask 13 defines the lateral extent of the p-dopedGroup III nitride material 18. The p-doped Group III nitride material 18may comprise GaN or AlGaN doped with magnesium. The hard mask 13 is usedto define the lateral extent of the recess 15 and the lateral extent ofthe p doped Group III nitride material 18. Therefore, the methodprovides a direct alignment of the p-doped Group III nitride material 18onto the recess 15. This embodiment may be used for a gate recess 15having a base 16 in the Group III barrier layer 10 such that at least aportion of the barrier layer 10 and the interface between the barrierlayer 10 and the channel layer 11 is positioned vertically below thebase 16 of the recess 15. A gate metal may then be deposited onto thep-doped Group III nitride material 18 at a position above the recess 15.The structure illustrated in FIG. 1c may be used to form a gatestructure in a Group III nitride-based transistor device, as isillustrated in FIG. 3 for example. The hard mask 13 and in particularthe opening 14 in the hard mask 13 is used to define and limit thelateral extent of the p-doped Group III nitride material 18 that formspart of the gate structure of the Group III nitride-based transistordevice. The p-doped Group III nitride material 18 is formed only in therecess 15 and at the position in which a gate structure is to be formed.

In some embodiments, the p-doped Group III nitride material 18 isselectively formed in the recess 15 using the hard mask 13 as aselective regrowth mask during regrowth and formation of the p-dopedGroup III nitride material 18. In these embodiments, the barrier layer10 may be entirely covered by the hard mask 13 so that the opening 14 oropenings 14 in the hard mask 13 expose only portions of the barrierlayer 10 on which a gate structure is to be formed.

In some non-illustrated embodiments, the p-doped Group III nitridematerial 18 may entirely fill the recess 15 in the barrier layer 10 suchthat its upper surface is positioned above the upper surface of thebarrier layer 10 and is bounded by the hard mask 13. After removal ofthe hard mask 13, the p-doped Group III nitride material 18 may protrudefrom the upper surface of the barrier layer 10. In some embodiments, therecess 15 and the opening 14 in the hard mask 13 are filled with thep-doped material 18. In some embodiments, the p-doped material 18 alsoextends over the upper surface of the hard mask 13.

FIG. 1d illustrates an embodiment, in which a second Group III nitridebarrier layer 19 is formed on the base 16 and on the sidewalls 17 of therecess 15 and then the p-doped Group III nitride material 18 is formedon the second Group III nitride barrier layer 19 with the hard mask 13in place on the upper surface 26 of the first Group III nitride barrierlayer 10. The hard mask 13 with its opening 14 are used to define thelateral extent of the second Group III nitride barrier layer 19 and thep-doped Group III nitride material 18. Therefore, the method provides adirect alignment of the, second Group III nitride barrier layer 19 andthe p-doped Group III nitride material 18 onto the recess 15. A secondGroup III nitride barrier layer 19 lining the gate recess 15 can be usedin embodiments in which the base 16 of the recess is positioned in thechannel layer 11.

The p-doped Group III nitride material 18 and the second Group IIInitride barrier layer 19 may be selectively formed in the recess 15. Agate metal may be formed on the p-doped Group III nitride material 16 ata position above the recess 15.

In some non-illustrated embodiments, the second Group III nitridebarrier layer 19 extends on the sidewalls 17 through the entirethickness of the barrier layer 10. The upper portion of the second GroupIII nitride barrier layer 19 may be bounded by the hard mask 13. Thep-doped material 18 may also have a depth such that its upper surface ispositioned substantially coplanar with the interface between the barrierlayer 10 and the hard mask 13. In some embodiments, the p-doped material18 has a depth such that its upper surface is positioned above theinterface between the barrier layer 10 and the hard mask 13 so that thep-doped material 18 protrudes above the upper surface of the barrierlayer 10 after the removal of the hard mask 13.

In some embodiments, an extension to one or both sides of the p-dopedGroup III nitride material 18 is formed so that the upper portion of thep-doped Group III nitride material 18 has a larger lateral extent thanthe base of the p-doped Group III nitride material 18. To form thesestructures, the method can continue as illustrated in FIG. 1e afterforming the recess 15 with the structure illustrated in FIG. 1 b.

After forming the recess 15, which may have a depth such that the base16 is positioned in either the first Group III nitride barrier layer 10or in the Group III nitride channel layer 11, a width of the opening 14in the hard mask 13 is increased, exposing a portion of the first GroupIII nitride barrier layer 10 that is positioned adjacent at least onesidewall 17 of the recess 15. In the embodiment illustrated in FIG. 1e ,the width of the opening 14 is increased to form a widened opening 14′such that a first portion 20 and a second portion 21 of the first GroupIII nitride barrier layer 10 is exposed adjacent opposing sidewalls 17,17′ of the recess 15.

In some embodiments, the widened opening 14′ is substantiallysymmetrically or concentrically arranged with respect to the opening 14and the recess 15 such that the first portion 20 and the second portion21 of the first Group III nitride barrier layer 10 that is exposedadjacent opposing sidewalls 17, 17′ of the recess 15 substantially thesame length. In some embodiments, the length of the first portion 20 andthe second portion 21 may differ such that the widened opening 14′ isasymmetrically positioned with respect to the recess 15.

The p-doped Group III nitride material 18 is then formed. In theembodiment illustrated in FIG. 1f , the p-doped Group III nitridematerial 18 fills the recess 15 and has portions positioned on the GroupIII nitride barrier layer 10 which extend in opposing directions fromthe side wall 17 of the recess 15 and form extensions 22, 23. Thewidened opening 14′ in the hard mask 13 defines the lateral extent ofthe p-doped Group III nitride material 18 as illustrated in FIG. 1f .The width of the exposed portions 20, 21 corresponds to the desiredlength of the extension(s) of the subsequently formed gate structureand, in particular, the extensions of the p-doped Group III nitridematerial 18 which is inserted into the formed in the recess 15 and onthe exposed portions 20, 21 of the first Group III nitride barrier layer10.

Each extension 22, 23 has a strip-like form in plan view for embodimentsin which the recess 15 has a strip-like form in plan view. The p-dopedGroup III nitride material 18 has an upper portion having a greaterlateral extent than the lower portion which is bounded by the side walls17 of the trench 15. A gate metal may then be formed on the p-dopedGroup III nitride material 18.

In embodiments including a second Group III barrier layer 19 positionedin the recess 15, as illustrated in FIG. 1g , the hard mask 13 with thewidened opening 14′ is also used to define the lateral extent of thesecond Group III nitride barrier layer 19. The second Group III nitridebarrier layer 19 is formed on the base 16 and sidewall 17 of the recess15 and onto the exposed portions 20, 21 of the first Group III nitridebarrier layer 10. The Group III nitride barrier layer 19 has first andsecond extensions 24, 25 defined by the widened opening 14′. The p-dopedGroup III nitride material 18 is formed on the second Group III nitridebarrier layer 19 and has a lateral extent which is also defined by theopening 14′ such that the p-doped Group III nitride material 18 also hasfirst and second extensions 22, 23 having a length corresponding to thefirst and second extensions 24, 25 of the second Group III nitridebarrier layer 19 which in turn correspond to the lateral extent of theopening 14′. A gate metal may be formed on the p-doped Group III nitridematerial 18.

The width of the opening 14 may be increased adjacent one side of therecess by a length L such that L is greater than 0 nm and less than 200nm, or between 10 and 100 nm. In some embodiments, the width of theopening 14 may be increased adjacent only one side of the recess by alength L such that L is greater than 0 nm and less than 200 nm, orbetween 10 and 100 nm, and such that there is no extension revealing aportion of the Group II nitride barrier layer 10 positioned adjacent theopposing side of the recess 18.

In some embodiments, the width of the opening 14 is increased by alength adjacent two opposing sides of the recess 15, whereby the lengthL₁ on a first side of the recess 15 may be different from the length L₂on a second opposing side of the recess 15. In some embodiments, theopening 14 is increased by a length L_(s) on a source side of the recess15 and by length L_(d) on a drain side the recess, whereby the lengthL_(s) is greater than the length L_(d).

In some embodiments, the method continues by forming a gate metal on thep-doped Group III nitride to form the gate structure for the Group IInitride-based transistor device and by forming a source contact and adrain contact on the first Group III nitride barrier layer 10 onopposing sides of the recess 15 in order to fabricate a Group IIInitride transistor device. In some embodiments, the transistor device isa High Electron Mobility Transistor (HEMT) and is an enhancement modedevice.

Surprisingly, and contrary to the expected behaviour of a field plate, ashorter extension for example an extension of less than 400 nm, or evenno extension, in the direction of the drain has been found to reducegate leakage current, whereas lengths of 400 nm or more have been foundto increase the gate leakage current.

FIGS. 2a to 2e illustrate a method for fabricating a gate structure fora Group III nitride-based transistor device. This method may be usefulfor fabricating a gate structure in which the p-doped material 18includes an extension which is has a greater length, in particular anextension on the source side of the gate which has a greater length.

FIG. 2a illustrates a first Group III nitride barrier layer 10 arrangedon a Group III nitride channel layer 11 and a hard mask 13 arranged onthe first major surface 26 of the first Group III nitride barrier layer10. The hard mask 13 has an opening 14 which is used to define thelateral extent of a recess 15 which extends from the upper surface 26 ofthe first Group III nitride barrier layer 10 into the first Group IIInitride barrier layer 10. In the embodiment illustrated in FIGS. 2a to2e , the recess 15 has a base 16 which is positioned within the GroupIII nitride channel layer 11. However, the method may also be used forrecess 15 having a smaller depth in which the base 16 of the recess 15is positioned within the first Group III nitride barrier layer 10.

After the formation of the recess 15, the opening 14 of the hard mask 13may be widened, as illustrated in FIG. 2b , to from a widened opening14′. The hard mask 13 may be etched at positions adjacent the opening 14in order to increase the size of the opening 14 to form the widenedopening 14′.

In some embodiments, the opening 14 may be widened such that a firstportion 20 of the first Group III nitride barrier layer 10 is exposedfrom the hard mask 13 adjacent a first side 17 of the recess 15. In someembodiments, the opening 14 is widened such that a second portion 21 ofthe Group III nitride barrier layer 10 positioned adjacent a secondopposing side 17′ of the recess 15 is uncovered by the widened opening14′.

In some embodiments, the opening 14 may be widened such that a firstportion 20 of the first Group III nitride barrier layer 10 is exposedfrom the hard mask 13 adjacent a first side 17 of the recess 15 and suchthat a second portion 21 of the Group III nitride barrier layer 10positioned adjacent a second opposing side 17′ of the recess 15 isuncovered by the widened opening 14′. The length of the first portion 20and second portion 21 may the substantially the same. The structureillustrated in FIG. 2b may correspond to that illustrated in FIG. 1 e.

In some embodiments, the formation of the widened opening 14′ as isillustrated in FIG. 2b may be omitted and after the formation of therecess 15 as illustrate in FIG. 2a , the method continues by using anadditional mask 30 to structure the hard mask 13, as illustrated in FIG.2 c.

As illustrated in FIG. 2c , the method then continues by using anadditional mask 30 to structure the hard mask 13 which may include theopening 14 or the widened opening 14′. The additional mask 30 has anopening 31 that positioned relative to the widened opening 14′ in thehard mask 13 so that a portion of the hard mask 13 adjacent the firstside wall 17 of the recess 15 is positioned in the opening 31. Thisportion of the hard mask 13 is removed, for example by etching tofurther increase the width of the widened opening 14′ and to increasethe length La of the exposed portion 20′ of the upper surface 26 of theGroup III nitride layer 10, as is illustrated in FIG. 2d . The opening31 may be positioned relative to the widened opening 14′ such that thesecond extension 21 and the recess 15 remain substantially the samesize. The mask 30 is then removed.

The hard mask 13 with the doubly widened opening 14″ is used to definethe lateral extent of the p-doped conductive material 18 which isselectively formed in the recess 15 and on the first and second exposedportions 20′, 21 of the first Group III nitride barrier layer 10 to formfirst and second extensions 22, 23. The p-doped conductive material 18may be selectively formed in the recess 15 and on the first and secondexposed portions 20′, 21 of the first Group III nitride barrier layer10.

In some embodiments, for example embodiments in which the opening 14 isnot widened before application of the additional mask 30 and formationof the widened opening as defined by the opening 31 in the mask 30,after the removal of the mask 30, this widened opening is furtherwidened. The method illustrated in FIG. 2b can be performed after themethod illustrated in FIGS. 2c and 2d is performed.

In some embodiments, such as that illustrated in FIG. 2e , a secondGroup III nitride barrier layer 19 is formed onto the base 16 andsidewalls 17, 17′ of the recess 15 and onto the exposed portions 20′ and21 of the first Group III nitride barrier layer 10. The opening 14″ inthe hard mask 13 defines the lateral extent of the second Group IIInitride barrier layer 32 and the extensions 32, 33 of the second GroupIII nitride barrier layer 19 positioned on the upper surface 26 of thefirst Group III nitride barrier layer 10.

The p-doped Group III nitride material 18 is then in formed on thesecond Group III nitride barrier layer 19 such that p-doped Group IIInitride material 18 fills the recess 15 and is positioned on theextensions 32, 33 of the second Group III nitride barrier layer 19. Thep-doped Group III nitride material 18 has extensions 34, 35 having alateral extent defined by the opening 14″ in the hard mask 13 andtherefore having the same lateral extent as the second Group III nitridebarrier layer 19.

A gate metal is deposited onto the p-doped Group III nitride material18. The first extensions 32, 34 have a greater length than the secondextensions 33, 35, respectively, and may extend towards the source inthe transistor structure. A two-stage process to increase the width ofthe opening 14 in the hard mask 13, that is used to form the recess 15,may be used to form the opening 14″ that is used to define the lateralextent of the first extensions 32, 34. This two-stage process may beuseful if the extensions are asymmetrical, i.e. the first extensions 32,34 adjacent a first sidewall 17 of the recess 15 have a greater lengththan the second extensions 33, 35 adjacent the opposing side wall 17′ ofthe recess 15. In some embodiments, the two-stage process may be used ifthe recess has only a single extension, i.e. an extension on the sourceside of the recess 15.

Group III nitride transistor devices with gate structures, which may befabricated using the methods disclosed in FIGS. 1a to 1g and FIGS. 2a to2e , will now be described.

FIG. 3 illustrates a Group III nitride-based transistor device 40 with agate structure 41 according to an embodiment. The Group IIInitride-based transistor device 40 includes a first Group III nitridebarrier layer 42 arranged on a Group III nitride channel layer 43. TheGroup III nitride barrier layer 42 and the Group III nitride channellayer 43 have differing bandgaps and form a heterojunction 44 capable ofsupporting a two-dimensional charge gas which is indicated schematicallyin the drawings by a dashed line. The Group III nitride barrier layer 42may be formed of AlGaN and the Group III nitride channel layer 43 may beformed of GaN. The Group III nitride-based transistor device 40 alsoincludes a source 45, gate 46 and a drain 47 on an upper surface 48 ofthe Group III nitride barrier layer 42. The source 45, gate 46 and drain47 may be formed of one or more metals or alloys.

The gate structure 41 of the Group III nitride-based transistor device40 includes a gate recess 50 which extends from the upper surface 48 ofthe first Group III nitride barrier layer 42 into the first Group IIInitride barrier layer 42. In the embodiment illustrated in FIG. 3, therecess 50 has a depth from the first major surface 48 such that a base51 of the recess 50 is positioned in and formed by the material of thefirst Group III nitride barrier layer 42. A p-doped Group III nitridematerial 53 is arranged in the gate recess 50 and the metal gate 46 ispositioned on the p-doped Group III nitride material 53.

The transistor device 40 may also include one or more insulating layers49 which may act as passivation layers on the upper surface 48 of thebarrier layer 42. The insulating layers 49 include openings to allow thesource 45 and drain 47 to extend to the Group III nitride barrier layer42. The insulating layer(s) 49 may include SiN.

The metal gate 46 may be positioned in an opening in the insulatinglayer(s) 49 and has a lateral extent that is less than the lateralextent of the p-doped Group III nitride material 18 positioned on theupper surface 26. The p-doped Group III nitride material 53 extendsbetween the base 51 of the recess 50 and the gate metal 46.

The p-doped doped Group III nitride material 52 arranged between themetal gate 46 and the first Group III nitride barrier layer 42 producesan enhancement mode device which is normally off. In the embodimentillustrated in FIG. 3, the recess 50 has sidewalls 52 which extendsubstantially perpendicularly to the first major surface 48. The p-dopeddoped Group III nitride material 53 is confined within the recess 50 andhas an upper surface which is positioned within the recess 50 such thatit is spaced at a distance from the upper surface 48. The p-doped GroupIII nitride material 53 may be GaN or AlGaN doped with magnesium.

The Group III nitride channel layer 43 and the Group III nitride channellayer 42 may be formed on a support substrate 54. The support substrate54 may include sapphire or silicon, for example. The Group III nitridechannel layer 43 and Group III nitride barrier layer 42 may beepitaxially formed on the substrate 54. The support substrate 54 mayinclude a surface 55 which is capable of supporting the epitaxial growthof one or more Group III nitrides, for example the Group III nitridechannel layer 43 and Group III nitride channel layer 42. A bufferstructure, which is not illustrated in the drawings, may be positionedbetween an upper surface 55 of the substrate 54 and the Group IIInitride channel layer 43.

A typical buffer structure for a silicon substrate includes a AlNstarting layer, which may have a thickness of several 100 nm, on thesilicon substrate followed by a Al_(x)Ga_((1-x))N layer sequence, thethickness again being several 100 nm's for each layer, whereby the Alcontent of about 50-75% is decreased down to 10-25% before the GaN layerof AlGaN back barrier is grown. Alternatively, a superlattice buffer canbe used. Again, an AlN starting layer on the silicon substrate is used.Depending on the chosen superlattice, a sequence of AlN andAl_(x)Ga_((1-x))N pairs is grown, where the thickness of the AlN layerand Al_(x)Ga_((1-x))N is in the range of 5-15 nm. Depending on thedesired breakdown voltage the superlattice may include between 20 and100 pairs. Alternatively, an Al_(x)Ga_((1-x))N layer sequence asdescribed above can be used in combination with the above mentionedsuperlattice.

FIG. 4 illustrates a Group III nitride-based transistor device 60including a gate structure 61 according to an embodiment. The Group IIInitride-based transistor device 60 includes the substrate 54, Group IIInitride channel layer 43 Group III nitride barrier layer 42 arranged onthe Group III nitride channel layer 43 and source 45, gate 46 and drain47 as in the embodiment illustrated in FIG. 3. The gate structure 31includes a gate recess 50 and a p-doped Group III nitride material 53arranged in the recess 50 as in the embodiment illustrated in FIG. 3.

The recess 50 of the embodiment illustrated in FIG. 4 has a greaterdepth such that the base 51 of the recess 50 is positioned within andformed by the Group III channel layer 43. The recess 50 extends throughthe entire thickness of the Group III barrier layer 42. The gatestructure 61 includes a second Group III nitride barrier layer 62 whichlines the sidewalls 52 and base 51 of the recess 50. The second GroupIII nitride barrier layer 62 may be formed of AlGaN and may have thesame or a different composition as the first Group III nitride barrierlayer 42. The p-doped Group III nitride material 53 is positioned in therecess and is in contact with the second barrier Group III nitridebarrier layer 62. The lateral extent of the second Group III nitridebarrier layer 62 and the p-doped Group III nitride material 53 isdefined by the lateral extent of the recess 50.

In some embodiments, a second barrier layer 62 which lines the recess 50may be used for a recess having a base positioned within the first GroupIII nitride barrier layer 42.

FIG. 5 illustrates a Group III nitride-based transistor 70 with a gatestructure 71 according to an embodiment. The gate structure 71 includesa recess 50 defined by sidewalls 52 and a base 51. The recess extendsfrom the upper surface 48 of the Group III nitride-based barrier layer42 and, as in the embodiment illustrated in FIG. 3, the base 51 of therecess 50 is positioned within and defined by the material of the GroupIII nitride barrier layer 42. The gate structure 51 also includesp-doped Group III nitride material 53 that is positioned in the recess50.

In the gate structure 71, the p-doped doped Group III nitride material53 is not only positioned in the recess 50 but also extends overadjoining regions of the upper surface 48 of the Group III nitridebarrier layer 42 on one side or on two opposing sides of the recess 50.In particular, the p-doped Group III nitride-based material 53 can beconsidered to have a first extension 72 having a length L₁ and secondextension 73 having a length L₂ whereby the first extension 72 extendstowards the drain 47 and the second extension 73 extends towards thesource 45. In some embodiments, the gate structure 71 may include onlyone extension and this one extension is the second extension 73 whichextends towards the source 45.

In some embodiments, the length L₁ of the first extension 72 lies withinthe range of 0 to 200 nm or 10 nm to 100 nm. This length L_(l) of thefirst extension 72, which extends in the direction of the drain 47, isshorter than the extension provided by a typical T-shaped recessed gatestructure which provides a field plate. Such field plate type extensionshave a length of at least 400 nm. However, surprisingly, and contrary tothe expected behaviour of a field plate, a shorter first extension 72 inthe direction of the drain 47 has been found to reduce gate leakagecurrent, whereas lengths of 400 nm or more have been found to increasethe gate leakage current. Without being bound by theory, one explanationfor this behaviour is that leakage into the gate is related to a holecurrent at the interface 74 between the Group III nitride barrier 42 andinsulation layer 49 which flows into the p-doped Group III nitride gatematerial 53.

In some embodiments, the second extension 73, extending towards thesource 45 and away from the first extension 72, has a length L₂ which isgreater than the length L₁ of the first extension 72. The length L₂ ofthe second extension 13 may lie in the range of 0 to 1000 nm, or 0 to500 nm, for example.

FIG. 6 illustrates a Group III nitride-based transistor device 80 whichhas a gate structure 81 having a recess 50 which extends from the uppersurface 48 through the Group III nitride barrier layer 42 such that thebase 51 of the recess 50 is positioned within and defined by the GroupIII nitride channel layer 43, as in the embodiment illustrated in FIG.4. The Group III nitride-based transistor device 80 also includes asecond Group III nitride barrier layer 62 which lines the base and side51 and sidewalls 52 of the recess 50 as in the embodiment illustrated inFIG. 4.

In the embodiment illustrated in FIG. 6, the gate structure 81 includesa second Group III nitride barrier layer 52, that not only lines thebase 51 and sidewalls 52 of the recess 50 but also extends over theupper surface 48 of the first Group III nitride barrier layer 62. Thesecond Group III nitride barrier layer 62 has a first extension 82 whichextends towards the drain 47 and a second extension 83 which extendstowards the source 45. The first and second extensions 82, 83 of thesecond Group III nitride barrier layer 62 have the same lateral extentas the first and second extensions 72, 73 of the p-doped Group IIInitride material 53.

The p-doped Group III nitride material 53 is arranged on the secondGroup III nitride barrier layer 62. In the embodiment illustrated inFIG. 6, the p-doped Group III nitride material 53 positioned on therecess 50 also has first and second extensions 72, 73 as in theembodiment illustrated in FIG. 5. The first and second extensions 82, 83of the second Group III nitride barrier layer 32 may have a length ofbetween 0 and 200 nm or 0 and 100 nm in the case the first extension 82and between 0 and 1000 nm or 0 and 500 nm for the second extension 83.In some embodiments, the second extension 83 extending towards thesource 45 is longer than the first extension 82 extending towards thedrain 47.

In some embodiments, the second barrier layer 62 which lines the recess50 may be used for a recess having a base positioned within the firstGroup III nitride barrier layer 42.

FIG. 7 illustrates a flow chart 90 of a method of fabricating a gatestructure for a Group III nitride-based transistor.

In block 91, a hard mask is formed on an upper surface of a first GroupIII nitride barrier layer that is arranged on a Group III nitridechannel layer, the first Group III nitride barrier layer and the GroupIII nitride channel layer having differing bandgaps and forming aheterojunction capable of supporting a two-dimensional charge gas, thehard mask having an opening. In block 92, at least a portion of thefirst Group III nitride barrier layer exposed in the opening of the hardmask is removed to form a recess having a base and side walls, therecess extending from the upper surface of the first Group III nitridebarrier layer into the first Group III nitride barrier layer. In block93, a p-doped Group III nitride material is formed in the recess, theopening defining the lateral extent of the p-doped Group III nitridematerial. In some embodiments, the p-doped Group III nitride material isselectively formed in the recess.

In some embodiments, the p-doped Group III nitride material 18 isselectively formed in the recess 15 using the hard mask 13 as aselective regrowth mask during regrowth and formation of the p-dopedGroup III nitride material 18. In these embodiments, the barrier layer10 is entirely covered by the hard mask 13 so that the opening 14 oropenings 14 in the hard mask 13 expose only portions of the barrierlayer 10 on which a gate structure is to be formed.

In some embodiments, such as that described with reference to FIGS. 8ato 8d , the hard mask 13 is structured to include an opening 14 thatexposes portions of the underlying barrier layer 10 on which a gatestructure is to be formed, as illustrated in FIG. 8a . FIG. 8a alsoillustrates the formation of a recess 15, in the barrier layer 10 and,optionally, the channel layer 11, as described with reference to FIGS.1a to 1g and FIG. 3. As illustrated in FIG. 8b , after the formation ofthe recess 15, the hard mask 13 is further structured to remove portionslaterally adjacent the recess 15 so as to expose other portions 101 ofthe barrier layer 10 from the hard mask 13.

These other regions 101 of the barrier layer 13 that are uncovered andexposed from the hard mask 13 may be located between the sourceelectrode and drain electrode of the transistor device and on top of theactive region of the transistor device or may be located peripherallyoutside of the source electrode and drain electrode and in the inactiveedge region of the transistor device.

The p-doped layer 18 is formed in the recess 15 and on the planarsurface of the barrier layer 10 in the regions 101 adjacent the recess15 that are uncovered by the hard mask 13, as illustrated in FIG. 8c .If the recess 15 extends into the channel layer 11, a second barrierlayer 19 can be formed on the sidewalls 17 and base 16 of the recess 15before the formation of the p-doped Group III nitride material 18. Therecess 15 and the opening 14 in the hard mask 13 defines the lateralextent of the portion of the p-doped Group III nitride material 18 forthe gate structure.

The portion of the p-doped Group III nitride material 18 positionedabove the recess 15 is covered by a further mask 100, for example aphoto resist. This mask 100 may expose the peripheral edges of the hardmask 13 and exposes the portions of the p-doped Group III nitridematerial 18 that are positioned on the planar surface of the barrierlaver 10 in the regions 101 adjacent the recess 15 and hard mask 13. Theexposed portions of the p-doped layer 18 can be removed, as illustratedin FIG. 8d , exposing the regions 101 of the barrier layer 10.

In these embodiments, the hard mask 13 is positioned only at thosepositions, i.e. immediately laterally adjacent the gate structure, whereit is used to define the lateral extent of the p-doped Group material 18in and immediately adjacent the recess 15 and the lateral extent of theGroup III nitride material for the gate structure of the Group IIInitride-based transistor device. This method may be used to simplify theformation of the p-doped Group III nitride material 18 and may be usedif the p-doped Group III nitride material 18 is formed by a regrowthtechnique.

FIGS. 9a to 9d illustrate a method in which dummy structures are formedfrom the p-doped Group III nitride material 18. These dummy structuresmay be formed in place of or in addition to the recess 15 and thep-doped Group III nitride material for the gate structure. The dummystructures may be formed on regions of the barrier layer 10 that arelaterally adjacent and spaced apart from the recess 15.

FIG. 9a illustrates an embodiment in which the hard mask 13 in formed soas to cover the barrier layer 10 and is structured to include an opening14 for a gate structure. The hard mask 13 extends over the remainder ofthe barrier layer 10. The portion of barrier layer 10 and optionally theunderlying channel layer 11 that is exposed in the opening 14 is removedto form a recess 15 for a gate structure.

The hard mask 13 is then further structured to define a further opening14′ on the planar surface of the barrier layer 10 that is positionedlaterally adjacent and spaced apart from the recess 15. The opening 14′defines the position and lateral extend of a dummy structure. Theremainder of the hard mask 13 is removed to uncover regions 101 of thebarrier layer 10. The regions of the hard mask 13 defining the openings14, 14′ have a width sufficient to define the opening 14, 14′. Thefurther opening 14′ may be positioned between the gate structure and theposition of the drain electrode or the source electrode or in theperipheral edge region of the final transistor device. Two or morefurther openings may also be provided. The p-doped Group III nitridematerial 18 is then formed in the opening 14 and in the recess 15 forforming the gate structure, in the opening 14′ for forming the dummystructure and on the regions 101 uncovered by the barrier layer 10.

FIG. 9b illustrates that the portions of the p-doped layer 18 that havea lateral extent defined by the openings 14; 14′ in the hard mask 13 arecovered by portions of a further structured resist layer 100. Thefurther structured resist layer 100 exposes the remainder of the p-dopedlayer 18. The regions of the p-doped Group III nitride material 18 thatare uncovered by the resist layer 100 are then removed, as illustratedin FIG. 9c , and then the resist layer 100 is removed, as illustrated inFIG. 9d . A dummy structure 102 is formed from the p-doped Group IIInitride material 18 that is positioned on the planar surface of thebarrier layer 10 at a position that is laterally adjacent and spacedapart from the recess 15 and the p-doped Group III nitride material 18for the gate structure that is positioned within the recess 15 or, inembodiments in which the recess 15 is not formed, from the Group IIInitride material 18 for the gate structure. In such a case, when therecess 15 is not formed, the p-doped Group III nitride material 18 forthe gate structure is formed similar to the dummy structure 102 on thesurface of the barrier layer 10 and may, insofar, be regarded as ap-doped Group III nitride material dummy structure 102 itself.

The lateral extent of the p-doped Group III nitride material 18 for thegate structure and for the dummy structure 102 is defined by the hardmask 13.

In some embodiments, the p-doped Group III nitride material 18 for thegate structure further extends onto the barrier layer 10 in regionsimmediately adjacent the recess 15, as, for example, in the embodimentsillustrated in FIGS. 1g, 2e , 5 and 6. The dummy structure 102 can bepositioned on an active portion of the final Group III nitridetransistor device, for example between the gate and source or betweenthe gate and drain, or on an inactive portion, for example in theperipheral region of the device. More than one dummy structure may beformed, and one or more dummy structures may be positioned in the activeand/or inactive portions of the Group III nitride-based transistordevice. The dummy structure may be coupled to the drain as a drainconnected field plate-like structure.

In some embodiments, a Group III nitride-based transistor device isprovided that has a gate structure including a p-doped Group III nitridelayer under a metal gate. The p-doped Group III nitride layer isarranged in a gate recess and has an extension in the direction of thedrain that is smaller than the extension provided by a T-shape gaterecess in which the T provides a field plate. The length of theextension towards the drain lies within the range of 0 to 200 nm or 10nm to 100 nm. A typical T-shaped recess gate structure which provides afield plate has an extension with a length of at least 400 nm.

In some embodiments, the p-doped Group III nitride layer has a secondextension extending towards the source. The second extension has alength which is greater than the length of the extension that extendstowards the drain. This arrangement is the opposite to that commonlyused in which the field plate on the drain side of the gate is largerthan the field plate on the source side of the gate.

In some embodiments, a second Group III nitride barrier layer ispositioned in and lines the base and side walls of the recess. Thep-doped Group III nitride material is arranged on the second Group IIInitride barrier layer. In some embodiments, this second Group IIInitride barrier layer extends onto the upper surface of the first GroupIII nitride barrier and has one or two extensions. The second Group IIInitride barrier layer may have a first extension extending on the uppersurface of the first Group III nitride barrier layer towards the drain,the first extension having a length l_(d), and 0 nm<l_(d)≤200 nm. Insome embodiments, the second Group III nitride barrier layer has asecond extension extending on the upper surface of the first Group IIInitride barrier layer towards the source and the second extension has alength l_(s), and 0 nm<l_(s)≤500 nm and l_(s)>l_(d).

Surprisingly, and contrary to the expected behaviour of a field plate, ashorter extension, for example an extension of less than 400 nm, or evenno extension, in the direction of the drain has been found to reducegate leakage current, whereas lengths of 400 nm or more have been foundto increase the gate leakage current. Without being bound by theory, oneexplanation for this behaviour is that leakage into the gate is relatedto a hole current formed at the interface between the Group III nitridebarrier and an overlying insulation layer. This hole current is thoughtto flow into the p-doped Group III nitride gate material. Any potentialdifference between the channel and the gate structure would appear toenhance gate leakage. Consequently, reducing or eliminating anypotential difference between the channel and the gate structure wouldappear to be beneficial.

Spatially relative terms such as “under”, “below”, “lower”, “over”,“upper” and the like are used for ease of description to explain thepositioning of one element relative to a second element. These terms areintended to encompass different orientations of the device in additionto different orientations than those depicted in the figures. Further,terms such as “first”, “second”, and the like, are also used to describevarious elements, regions, sections, etc. and are also not intended tobe limiting. Like terms refer to like elements throughout thedescription.

As used herein, the terms “having”, “containing”, “including”,“comprising” and the like are open ended terms that indicate thepresence of stated elements or features, but do not preclude additionalelements or features. The articles “a”, “an” and “the” are intended toinclude the plural as well as the singular, unless the context clearlyindicates otherwise. It is to be understood that the features of thevarious embodiments described herein may be combined with each other,unless specifically noted otherwise.

Although specific embodiments have been illustrated and describedherein, it will be appreciated by those of ordinary skill in the artthat a variety of alternate and/or equivalent implementations may besubstituted for the specific embodiments shown and described withoutdeparting from the scope of the present invention. This application isintended to cover any adaptations or variations of the specificembodiments discussed herein. Therefore, it is intended that thisinvention be limited only by the claims and the equivalents thereof.

What is claimed is:
 1. A Group III nitride-based transistor device,comprising: a first Group III nitride barrier layer arranged on a GroupIII nitride channel layer, the first Group III nitride barrier layer andthe Group III nitride channel layer having differing bandgaps andforming a heterojunction capable of supporting a two-dimensional chargegas; a source, a gate and a drain on an upper surface of the first GroupIII nitride barrier layer; a gate recess extending from the uppersurface of the first Group III nitride barrier layer into the firstGroup III nitride barrier layer; and a p-doped Group III nitridematerial arranged in the gate recess and having a first extensionextending on the upper surface of the first Group III nitride barrierlayer towards the drain, the first extension having a length l_(d), and0 nm≤l_(d)≤200 nm.
 2. The Group III nitride-based transistor device ofclaim 1, wherein the p-doped Group III nitride material has a secondextension extending on the upper surface of the first Group III nitridebarrier layer towards the source and the second extension has a lengthl_(s), and wherein 0 nm<l_(s)≤500 nm and l_(s)>l_(d).
 3. The Group IIInitride-based transistor device of claim 1, further comprising a secondGroup III nitride barrier layer arranged on side walls and a base of thegate recess, wherein the second Group III nitride barrier layer has afirst extension extending on the upper surface of the first Group IIInitride barrier layer towards the drain, wherein the first extension hasa length l_(d), wherein 0 nm<l_(d)≤200 nm, and wherein the p-doped GroupIII nitride material is arranged on the second Group III nitride barrierlayer.
 4. The Group III nitride-based transistor device of claim 3,wherein the second Group III nitride barrier layer has a secondextension extending on the upper surface of the first Group III nitridebarrier layer towards the source and the second extension has a lengthl_(s), and wherein 0 nm<l_(s)≤500 nm and l_(s)>l_(d).
 5. The Group IIInitride-based transistor device of claim 1, wherein the gate recessextends into the Group III nitride channel layer such that the gaterecess has a base positioned in the Group III nitride channel layer. 6.The Group III nitride-based transistor device of claim 1, furthercomprising a gate metal on the p-doped Group III nitride material. 7.The Group III nitride-based transistor device of claim 1, wherein theGroup III nitride-based transistor device is a High Electron MobilityTransistor (HEMT) and is an enhancement mode device.
 8. The Group IIInitride-based transistor device of claim 1, wherein the gate recess hasa depth from the upper surface of the first Group III nitride barrierlayer such that a base of the recess is positioned in and formed by amaterial of the first Group III nitride barrier layer.
 9. The Group IIInitride-based transistor device of claim 1, wherein 10 nm≤l_(d)≤100 nm.10. A Group III nitride-based transistor device, comprising: a firstGroup III nitride barrier layer arranged on a Group III nitride channellayer, the first Group III nitride barrier layer and the Group IIInitride channel layer having differing bandgaps and forming aheterojunction capable of supporting a two-dimensional charge gas; asource, a gate and a drain on an upper surface of the first Group IIInitride barrier layer; a gate recess extending from the upper surface ofthe first Group III nitride barrier layer into the first Group IIInitride barrier layer; and a p-doped Group III nitride material arrangedin the gate recess, wherein the p-doped Group III nitride material has afirst extension extending on the upper surface of the first Group IIInitride barrier layer towards the drain, wherein the p-doped Group IIInitride material has a second extension extending on the upper surfaceof the first Group III nitride barrier layer towards the source, whereinthe second extension has a length which is greater than the length ofthe first extension.
 11. The Group III nitride-based transistor deviceof claim 10, further comprising a second Group III nitride barrier layerarranged on side walls and a base of the gate recess, wherein the secondGroup III nitride barrier layer has a first extension extending on theupper surface of the first Group III nitride barrier layer towards thedrain, and wherein the p-doped Group III nitride material is arranged onthe second Group III nitride barrier layer.
 12. The Group IIInitride-based transistor device of claim 11, wherein the second GroupIII nitride barrier layer has a second extension extending on the uppersurface of the first Group III nitride barrier layer towards the source,and wherein the second extension of the second Group III nitride barrierlayer has a length which is greater than the length of the firstextension of the second Group III nitride barrier layer.
 13. The GroupIII nitride-based transistor device of claim 10, wherein the gate recessextends into the Group III nitride channel layer such that the gaterecess has a base positioned in the Group III nitride channel layer. 14.The Group III nitride-based transistor device of claim 10, furthercomprising a gate metal on the p-doped Group III nitride material. 15.The Group III nitride-based transistor device of claim 10, wherein theGroup III nitride-based transistor device is a High Electron MobilityTransistor (HEMT) and is an enhancement mode device.
 16. The Group IIInitride-based transistor device of claim 10, wherein the first extensionof the p-doped Group III nitride material is less than 400 nm.
 17. TheGroup III nitride-based transistor device of claim 10, wherein the gaterecess has a depth from the upper surface of the first Group III nitridebarrier layer such that a base of the recess is positioned in and formedby a material of the first Group III nitride barrier layer.